The present invention notably relates to operations carried out in order to develop underground reservoirs containing hydrocarbons. Cementing generally requires injection by pumping of a cement slurry circulating in tubes from the surface. The aim is to cement the annular space defined between the outside of the casing pipe and the wellbore.
Conventional cement slurries generally have a density above 1800 kg/m3. Specific operations such as deep-sea drilling require high-performance grouts with a low density. The most conventional method for lightening the slurry consists in increasing the water/cement ratio (referred to as E/C in the description hereafter), which is however done to the detriment of the mechanical properties of the material once solidified. The minimum density of a slurry that can be reached by increasing the E/C ratio, without cohesion loss of the cement matrix, ranges, according to authors, between 1260 kg/m3 and 1450 kg/m3. Another way of lightening cement grouts consists in adding low-density materials to a conventional slurry formulation, such as coal powder, gilsonite, walnut shells.
Document U.S. Pat. No. 5,252,128 describes a mixture of cement, styrene/butadiene resin and alkyl phenol surfactant, containing 5 to 30% by weight of resin in relation to the cement, which allows good control of the filtration properties of the slurry.
Document U.S. Pat. No. 4,721,160 describes a mixture of cement and styrene/butadiene resin containing a very high proportion of water (70%), allowing to obtain a lightened cement whose density ranges between 1.2 and 1.6.
It has also been shown that an optimization of the grain size of the various constituents allows to obtain mixtures with a wide density range.
Patent WO-99/23,046 describes an oilwell cement of very low permeability and high mechanical strength based on cement, silica and microsilica. The slurries thus obtained have a density of about 2.3.
The object of the present invention is to associate with cement particles a resin (VASA) obtained by polymerization from vinyl aromatic, alkyl acrylate, acrylic type monomers, more or less crosslinked, of density 1.03, and with a grain size below 100 xcexcm (80% of the particles ranging between 0.1 and 100 xcexcm).
This VASA resin is organosoluble, i.e. the particles do not solubilize in the slurry water, but they remain in form of a dispersion mixed with the other particles. The VASA resin is notably characterized by:
a glass-transition temperature above 55xc2x0 C.,
when the resin is in solution in xylene at 10%, this solution has a Brookfield viscosity above 80 Pa.s.
According to the invention, the VASA resin acts as a particulate filler that is chemically inert towards the other constituents of the slurry: binders, additives, fillers, water.
It can preferably be a  less than  less than Pliolite greater than  greater than (trademark) type resin marketed by Good Year. This resin is used in the examples described hereafter.
At least one dispersing polymer derivative or, in particular, a hydrophilic/hydrophobic type dispersing polymeric additive is also preferably associated. This allows lightening of the mixture, as well as perfect dispersion of the organophilic particles in the other particles, and good control of the mixture rheology.
The dispersing polymer can be a polymer with hydrophilic (Hy) and hydrophobic (Hb) units in aqueous solution, the hydrophobic units (Hb) containing C1 to C30 alkyl, aryl, alkyl-aryl groups, the polymer having the following structure: xe2x80x94(Hb)xe2x80x94Hy)xe2x80x94 with a statistical distribution, and
Hy has the following form: 
xe2x80x83where R5 is H or CH3, Z1 is COOH or CONH2 or CONHR1SO3xe2x88x92 or CONHRxe2x80x31, Rxe2x80x31 is CH3;
Hb has the following form: 
xe2x80x83where Rxe2x80x25 is H or CH3 and Z2 is COOR7, C6H4SO3H, COORxe2x80x21, CONR1Rxe2x80x21 or CONR1R7, R7 being a non-ionic surfactant consisting of an alkyl polyoxyethylene chain, R1 is H or a C1-C30 alkyl, aryl or alkyl-aryl radical, and Rxe2x80x21 is a C1-C30 alkyl, aryl or alkyl-aryl radical.
According to a variant, the polymer can have a molecular mass ranging between 104 and 5 106 daltons and a proportion of hydrophobic units Hb ranging between 0.5 and 60%.
The dispersing polymer can be selected from the group consisting of:
HMPAM where R5 is H and Z1 is CONH2, Rxe2x80x25=CH3, Z2 is COORxe2x80x21 with Rxe2x80x21=C9H19,
S1 where R5 is H and Z1 is CONH2, Rxe2x80x25=H and Z2 is C6H4SO3H,
HB1 where R5 is H, Z1 is COOH, Rxe2x80x25 is H and Z2 is COORxe2x80x21 with Rxe2x80x21 is C4.
In particular, the polymer called HMPAM can have a molecular mass ranging between 5 105 and 2 106 daltons, and a proportion of hydrophobic units (Hb) ranging between 0.5 and 3%.
Polymer S1, an acrylamide (Hy)/styrene sulfonate (Hb) copolymer, branched or not, according to the description above, can have a molar ratio of about 50/50 and a molar mass ranging between 5 105 and 5 106 daltons. If it is branched, it is referred to as S2. The branching agent used in this case can be N, Nxe2x80x2 methylene bis acrylamide MBA.
Polymer HB1, an acrylate (Hy)/butyl acrylate (Hb) copolymer, where R5 is H, Z1 is COOH, Rxe2x80x25 is H and Z2 is COORxe2x80x21 where Rxe2x80x21 is C4, can comprise about 80% acrylate units, and its molecular mass can range between 104 and 5 104 daltons.
The slurry according to the invention can have the following composition: 10 to 20% silica, 10 to 15% microsilica, 30 to 60% VASA resin, 0.5 to 5% hydrophobic polymer, 30 to 50% water, these percentages being expressed in relation to the weight of cement.
Preferably, the slurry can comprise about 15% silica, 12% microsilica, 50% VASA resin, 1 to 3% hydrophobic polymer and 40 to 45% water.
In a variant, the slurry can have the following composition: 10 to 20% silica, 10 to 15% microsilica, 30 to 60% VASA resin, 80 to 95% water, and no dispersing polymer.